Field of the Invention
The present invention relates to an optical modulator and in particular, to an optical modulator which is provided with a relay substrate which performs a relay between a plurality of conductors (for example, lead pins) for inputting a high-frequency signal, which are provided in a housing, and electrodes of an optical modulation element, and an optical transmission apparatus using the optical modulator.
Description of Related Art
In high-frequency and high-capacity optical fiber communication systems, optical modulators having a waveguide-type optical modulation element incorporated therein are frequently used. Among them, an optical modulation element in which LiNbO3 (hereinafter, also referred to as LN) having an electro-optic effect is used for a substrate can realize an optical modulation characteristic with a small light loss and a wide band, and therefore, it is widely used for a high-frequency and high-capacity optical fiber communication system.
The optical modulation element using the LN substrate is provided with a Mach-Zehnder optical waveguide, a RF electrode forapplyinga high-frequency signal, which is amodulation signal, to the optical waveguide, and a bias electrode for performing a variety of adjustments in order to favorably maintain a modulation characteristic in the waveguide. Then, these electrodes provided in the optical modulation element are connected to a circuit board, on which an electronic circuit for causing the optical modulator to perform a modulation operation is mounted, via lead pins or connectors provided in a housing of the optical modulator, which houses the optical modulation element.
As a modulation form in the optical fiber communication system, in response to the recent trend toward an increase in transmission capacity, multilevel modulation such as Quadrature Phase Shift Keying (QPSK) or Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK), or a transmission format with polarization multiplexing incorporated into the multilevel modulation is the main stream and is used in a core optical transmission network. However, it is also being introduced into a metro network.
An optical modulator which performs QPSK modulation (a QPSK optical modulator), or an optical modulator which performs DP-QPSK modulation (a DP-QPSK optical modulators) is provided with a plurality of Mach-Zehnder optical waveguides having a nested structure which is called a so-called nested type, and is also provided with a plurality of high-frequency signal electrodes and a plurality of bias electrodes (refer to, for example, Japanese Laid-open Patent Publication No. 2016-109941). Therefore, the size of a housing of the optical modulator tends to be larger. However, in recent years, conversely, a demand for the downsizing of the modulator has increased.
As a measure responding to this downsizing demand, in the related art, an optical modulator which enables electrical connection with an external circuit board by replacing a push-on type coaxial connector provided in a housing of an optical modulator as an interface of the RF electrode with lead pins similar to an interface for a bias electrode and a flexible printed circuit (FPC) which is electrically connected to these lead pins has been proposed (refer to, for example, Japanese Laid-open Patent Publication No. 2016-109941).
For example, in the DP-QPSK optical modulator, an optical modulation element which is configured of four Mach-Zehnder optical waveguides each having a RF electrode is used. In this case, if four push-on type coaxial connectors are provided in a housing of an optical modulator, an increase in the size of the housing is inevitable. However, if lead pins and an FPC are used instead of the coaxial connector, downsizing becomes possible.
Further, the lead pins of the housing of the optical modulator and a circuit board on which an electronic circuit (a drive circuit) for causing the optical modulator to perform a modulation operation is mounted are connected to each other through the FPC, and therefore, it is not necessary to perform the excess length treatment of a coaxial cable which has been used in the related art, and it is possible to reduce the mounting space of the optical modulator in an optical transmission apparatus.
In such an optical modulator having the lead pins for inputting a high-frequency electrical signal provided in the housing, in general, the lead pins and the electrodes of the optical modulation element housed in the housing are connected to each other through a relay substrate disposed in the housing (refer to, for example, Japanese Laid-open Patent Publication No. 2016-109941).
FIG. 13A, FIG. 13B, and FIG. 13C are diagrams showing an example of the configuration of the optical modulator of the related art as described above. Here, FIG. 13A is a plan view showing the configuration of an optical modulator 1300 of the related art mounted on a circuit board 1330, FIG. 13B is a side view of the optical modulator 1300 of the related art, and FIG. 13C is a bottom view of the optical modulator 1300 of the related art. The optical modulator 1300 is provided with an optical modulation element 1302, a housing 1304 which houses the optical modulation element 1302, a flexible printed circuit (FPC) 1306, an optical fiber 1308 to input light to the optical modulation element 1302, and an optical fiber 1310 which leads light which is output from the optical modulation element 1302 to the outside of the housing 1304.
The optical modulation element 1302 is a DP-QPSK optical modulator which is provided with four Mach-Zehnder optical waveguides provided on, for example, an LN substrate, and four high-frequency electrodes (RF electrodes) 1312a, 1312b, 1312c, and 1312d which are respectively provided on the Mach-Zehnder optical waveguides and modulate light waves propagating through the optical waveguides.
The housing 1304 is configured of a case 1314a to which the optical modulation element 1302 is fixed, and a cover 1314b. In order to facilitate the understanding of the configuration of the inside of the housing 1304, in FIG. 13A, only a part of the cover 1314b is shown on the left side in the drawing.
The case 1304a is provided with four lead pins 1316a, 1316b, 1316c, and 1316d. The lead pins 1316a, 1316b, 1316c, and 1316d are sealed with glass sealing portions 1400a, 1400b, 1400c, and 1400d (described later), extend from the bottom surface (the surface shown in FIG. 13C) of the housing 1304 to the outside, and are connected to through-holes formed on the FPC 1306 with solder and the like.
One end of each of the lead pins 1316a, 1316b, 1316c, and 1316d is electrically connected to one end of each of the RF electrodes (i.e., signal electrode) 1312a, 1312b, 1312c, and 1312d of the optical modulation element 1302 through a relay substrate 1318.
The other end of each of the RF electrodes 1312a, 1312b, 1312c, and 1312d is electrically terminated by a terminator 1320.
FIG. 14A is a partial detail view of a portion F of the optical modulator 1300 shown in FIG. 13A, and FIG. 14B is a cross-sectional view of the optical modulator 1300 taken along line XIVB-XIVB in FIG. 13A and viewed in a direction of an arrow. The lead pins 1316a, 1316b, 1316c, and 1316d extend from the inside of the housing 1304 to the outside of the housing 1304 through the glass sealing portions 1400a, 1400b, 1400c, and 1400d provided in the case 1314a, protrude from the lower surface (the surface shown in FIG. 13C) of the housing 1304, and are solder-fixed to the through-holes of the FPC 1306.
The lead pins 1316a, 1316b, 1316c, and 1316d are disposed in the vicinity of a side (a lead pin-side edge 1410) on the lower side of the relay substrate 1318 in FIG. 14A (the left side of the relay substrate 1318 in FIG. 14B), and are electrically connected to conductor patterns 1402a, 1402b, 1402c, and 1402d provided on the relay substrate 1318 by solders 1408a, 1408b, 1408c, and 1408d, respectively.
Further, the conductor patterns 1402a, 1402b, 1402c, and 1402d are electrically connected to the RF electrodes 1312a, 1312b, 1312c, and 1312d of a lower end portion of the optical modulation element 1302 (the left end of the optical modulation element 1302 in FIG. 14B), disposed in the vicinity of a side (a modulator-side edge 1412) on the upper side of the relay substrate 1318 in FIG. 14A (the right side of the relay substrate 1318 in FIG. 14B), by, for example, gold wires 1406a, 1406b, 1406c, and 1406d, respectively.
The conductor patterns 1402a, 1402b, 1402c, and 1402d are usually configured as linear patterns parallel to each other in order to minimize a signal propagation loss and a skew (a propagation delay time difference) by minimizing the propagation distance of the high-frequency signal from the respective lead pins 1316a, 1316b, 1316c, and 1316d to the respective RF electrodes 1312a, 1312b, 1312c, and 1312d corresponding to the respective lead pins 1316a, 1316b, 1316c, and 1316d. Therefore, the optical modulator 1300 is configured such that the interval between the respective lead pins 1316a, 1316b, 1316c, and 1316d is the same as the interval between the respective RF electrodes 1312a, 1312b, 1312c, and 1312d. 
Further, the conductor patterns 1402a, 1402b, 1402c, and 1402d are, for example, grounded coplanar lines, and ground patterns 1418a, 1418b, 1418c, 1418d, and 1418e are provided so as to interpose each of the conductor patterns 1402a, 1402b, 1402c, and 1402d therebetween on the front surface (the surface on which the conductor pattern 1402a or the like shown in FIG. 14A is provided) of the relay substrate 1318. Further, each of the ground patterns 1418a, 1418b, 1418c, 1418d, and 1418e is electrically connected to a ground pattern (not shown) provided on the back surface of the relay substrate 1318 through a plurality of via holes 1420. In FIG. 14A, for ease of viewing, only the via holes provided in one ground pattern 1418a are denoted by reference numerals. However, all the circular portions shown in the regions of the other ground patterns 1418b, 1418c, 1418d, and 1418e are the same via holes.
The ground patterns 1418a, 1418b, 1418c, 1418d, and 1418e are electrically connected to ground electrodes (not shown) on the optical modulation element 1302 by gold wires 1412a, 1412b, 1412c, 1412d, and 1412e, respectively.
Here, in general, an electrical signal which is input from the lead pin 1316a or the like sealed by the glass sealing portion 1400a or the like is a high-frequency signal (a microwave signal) of several tens of GHz. For this reason, the designed impedance (a designed value of characteristic impedance) of the lead pin 1316a or the like, the designed impedance of the conductor pattern 1402a or the like which is formed on the relay substrate 1318, and the designed impedance of the RF electrode 1312a or the like of the optical modulation element 1302 are set to, for example, the same value (for example, 50 Ω), whereby impedance matching is achieved. In this way, reflection or radiation of the high-frequency signal in a high-frequency transmission channel from the lead pin 1316a or the like to the RF electrode 1312a or the like of the optical modulation element 1302 through the conductor pattern 1402a or the like on the relay substrate 1318 is suppressed.
With the above configuration, in the optical modulator 1300, the high-frequency electrical signals input from conductor patterns 1332a, 1332b, 1332c, and 1332d (FIG. 13A) formed on the circuit board 1330 to the lead pins 1316a, 1316b, 1316c, and 1316d via the FPC 1306 are input to the RF electrodes 1312a, 1312b, 1312c, and 1312d of the optical modulation element 1302 via the relay substrate 1318.
However, even in the optical modulator 1300 in which impedance matching is achieved as described above, there is a case where a problem such as a noise signal component being superimposed on each RF electrode 1312a or the like of the optical modulation element 1302, so that a high-frequency characteristic such as an eye pattern extinction ratio or a jitter of the optical modulator 1300 deteriorates and the transmission characteristics of the optical transmission apparatus deteriorate, arises.
As a result of intensive studies on this problem, the inventors of the present invention have found that one cause of this problems is a phenomenon in which a high frequency propagating through one conductor pattern (1402a or the like) resonates by repeating reflection at both end portions of the relay substrate and the resonated high frequency resonates with another conductor pattern (1402b or the like), whereby a part of the power of the high frequency transits to another conductor pattern (hereinafter referred to as a resonance transition).
That is, a connection portion with the lead pin 1316a or the like at an end portion on one side of the relay substrate 1318 is a portion in which the propagation direction of the high frequency propagating through the lead pin 1316a or the like is curved by 90 degrees toward the conductor pattern 1402a or the like on the relay substrate 1318 (FIG. 14B), and even if the designed impedance is matched between the lead pin 1316a or the like and the conductor pattern 1402a or the like, it may be impossible to sufficiently suppress the reflection of the high frequency at the end portion on one side.
Further, also at a connection portion with the RF electrode 1312a or the like of the optical modulation element 1302 in an end portion on the other side of the relay substrate 1318, patterns (the conductor pattern 1402a or the like and the RF electrode 1312a or the like) respectively formed on two substrates having different dielectric constants, such as the relay substrate 1318 (for example, ceramic) and the substrate of the optical modulation element 1302 (for example, lithium niobate), are connected to each other through, for example, a wire with a space interposed therebetween. For this reason, even if the designed impedances of these patterns are set to be the same as each other, it is difficult to completely suppress reflection of the high frequency at the end portion on the other side.
As a result, due to the reflection of the high frequency occurring at both end portions of the relay substrate 1318, the high frequency propagating through the conductor patterns 1402a or the like has the maximum power at a specific resonance frequency which is determined by the electrical length of the conductor pattern 1402a or the like which is a distributed constant line. Then, the resonance frequency component having the maximum power returns to the electrical signal source side as a reflected wave, thereby making the operation of an external circuit (for example, a drive circuit which outputs a high-frequency electrical signal for each RF electrode 1312a or the like) unstable, or reaches the RF electrode 1312a or the like as a traveling wave (or a transmitted wave) and becomes noise.
In particular, in the relay substrate 1318 of the optical modulator 1300 of the related art, as described above, the conductor patterns 1402a, 1402b, 1402c, and 1402d are configured as linear patterns parallel to each other in order to minimize a signal propagation loss and a skew, and therefore, the resonance frequencies in the respective conductor patterns become substantially the same. As a result, if resonance occurs in one conductor pattern, the high-frequency component of the resonance frequency having the maximum power transits to another conductor pattern having substantially the same resonance frequency, and thus the resonance transition occurs.
Then, in a case in which such a resonance transition occurs, a resonance frequency component having a high power generated in one conductor pattern affects not only the operation of a corresponding RF electrode but also the operation of the other RF electrode through the resonance transition. For this reason, in particular, in the case of a device in which four RF electrodes perform an optical modulation operation in cooperation with each other, like the DP-QPSK modulator, the resonance frequency component having a high powder generated in the one conductor pattern appears as a synergetic effect of four noises generated in each of the four RF electrodes and deteriorates a high-frequency characteristic such as an eye pattern extinction ratio or a jitter of modulated light.
Further, such a resonance transition easily occurs when a plurality of high-frequency signals propagate in parallel within a narrow region, and the larger the power of the input high-frequency signal (for example, the amplitude of the high-frequency signal), the more easily the resonance transition occurs. For example, in the DP-QPSK modulator to which four high-frequency signals are input, a high-frequency signal having an amplitude of twice a half-wavelength voltage is input to each electrode, and therefore, in a configuration in which the interval between the lead pins is narrowed using an FPC, as described above, the input of a high-frequency signal having a high power concentrates on a narrow region, and thus an environment in which the resonance transition more easily occurs can be made.